Method for catalytic growth of nanotubes or nanofibers comprising a NiSi alloy diffusion barrier

ABSTRACT

The invention relates to a process for the growth of nanotubes or nanofibers on a substrate comprising at least an upper layer made of a first material, wherein: the formation, on the surface of the upper layer, of a barrier layer made of an alloy of the first material and of a second material, said alloy being stable at a first temperature; the formation of spots of catalyst that are made of the second material, on the surface of the alloy layer; and the growth of nanotubes or nanofibers at a second temperature below said first temperature. The alloy layer allows effective growth of nanotubes/nanofibers from catalyst spots on the surface of said alloy layer. This is because the alloy layer constitutes a diffusion barrier preventing the catalyst from diffusing into the growth substrate, which barrier is stable at the catalytic nanotube/nanofiber growth temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is based on International Application No.PCT/FR02/04155, filed on Dec. 3, 2002, which in turn corresponds to FR01/15647 filed on Dec. 4, 2001, and priority is hereby claimed under 35USC §119 based on these applications. Each of these applications arehereby incorporated by reference in their entirety into the presentapplication.

FIELD OF THE INVENTION

The field of the invention is that of nanotubes or nanofibers that maybe of the carbon, silicon or boron type or made of any other alloy basedon at least one of these components (for example SiC) and possiblycontaining nitrogen (SiN, BN, SiCN). Typically, these nanotubes ornanofibers have diameters ranging from a few nanometers to a few hundrednanometers over several microns in height.

DESCRIPTION OF RELATED ART

They are particularly beneficial for nanotechnology, composites, batteryelectrodes, energy storage, nanoelectronics, and field-emission devices.

As regards nanotechnology, the applications are in design and molecularengineering, nanotips (for metrology), actuators, robots, sensors, andtherefore MEMS (micro-electromechanical systems).

As regards energy storage, the applications are for fuel cells, whichuse the hydrogen storage properties of nanotubes, and alsosupercapacitors.

Nanoelectronics includes conventional electronic components (diodes,transistors, capacitors), molecular electronics, and future componentsin the case of future computers (carbon nanotube molecular computers).

In the case of field-emission devices, the applications are coldelectron sources for electron microscopy, analytical equipment using anelectron beam, nanolithography, electron tubes, ion motors and flatdisplay devices.

The growth of nanotubes/nanofibers on a substrate or a support iscarried out on catalyst aggregates of very small size (<100 nm) at atemperature generally above 500° C. and possibly exceeding 1000° C.

Conventionally, nanotubes or nanofibers are produced by growth fromsmall catalyst spots that may be defined by lithography. FIG. 1illustrates such a growth. Starting from a substrate 1, submicronapertures (preferably around 100 nm in size) are made in a resist 2(FIG. 1 a). Next, catalyst is deposited as a thin film 3, with athickness of less than about 10 nm (FIG. 1 b). After dissolving theresist (FIG. 1 c), catalyst spots having a diameter equivalent to thediameter of the apertures in the resist are obtained. The process thencontinues with the growth of nanotubes or nanofibers (FIG. 1 c).

The methods of preparation are the following: electrical discharge,pyrolysis, physical methods such as laser ablation and chemical methodssuch as CVD (chemical vapor deposition) or PECVD (plasma-enhanced CVD).

The method that seems best suited to the field-effect cathodeapplication is the PECVD method, which is assisted by DC plasma, RF(radiofrequency) plasma or microwave plasma. This method allowsnanotubes and nanofibers to be obtained that are oriented perpendicularto the substrate.

The nanotubes or nanofibers shown in all the figures of the applicationare drawn schematically. In contrast to the nanofibers, the nanotubesare hollow.

For example, in the case of carbon nanotubes, the diameter of thenanotubes is close to that of the catalyst particle. Owing to theelongate shape that this particle (see FIG. 1 d) made of material B(which may be C, SiC, BN, etc.) adopts, its diameter is smaller thanthat of the spots defined beforehand by lithography.

However, if during the step of raising the temperature of the substrateor support the catalyst aggregates diffuse into or are dissolved in thesubstrate or support, there will therefore be no nanotube/nanofibergrowth. It is therefore of paramount importance to deposit an effectivediffusion barrier prior to the depositing the catalyst. If the barrieris not very effective, the growth of nanotubes/nanofibers will be poorlycontrolled.

The division barriers currently used are generally silica (SiO₂) andtitanium nitride (TiN). SiO₂ is an excellent barrier but it is aninsulating material and therefore poorly suited in the case in which itis necessary to electrically connect the nanotubes. It should be notedthat SiO₂ may, however, be used as a very thin film (2-4 nm) and that,in this case, the current can flow by a tunnel effect. SiO₂ rapidlydegrades as the electrical current flows and it then loses itsinsulating properties. TiN is also an excellent diffusion barrier, butonly for nanotube growth temperatures below 700° C. This is becauseabove 700° C., the nitrogen constituent of the TiN undergoesexodiffusion and the material then loses its diffusion barrierproperties.

In this context, the invention proposes to use novel diffusion barriersdesigned for catalytic nanotube and nanofiber growth and suitable forcatalysts of the nickel, cobalt, iron, platinum, or yttrium type, orthose made of any other alloy based on at least one of these components.

SUMMARY OF THE INVENTION

More precisely, the subject of the invention is a process for the growthof nanotubes or nanofibers on a substrate comprising at least an upperlayer made of a first material, characterized in that it comprises:

-   -   the formation, on the surface of the upper layer, of a barrier        layer made of an alloy of the first material and of a second        material, said alloy being stable at a first temperature;    -   the formation of spots of catalyst that are made of the second        material, on the surface of the alloy layer; and    -   the growth of nanotubes or nanofibers at a second temperature        below said first temperature.

According to a variant of the invention, the formation of the barrierlayer comprises the deposition of a layer made of the second material onthe surface of the upper layer made of the first material, followed byannealing at said first temperature.

The invention thus consists in depositing a thin layer of secondmaterial constituting the catalyst on the upper layer of first materialand then in annealing at a temperature greater than or equal to thenanofiber/nanotube growth temperature. Thus, an alloy stable at theannealing temperature T_(a), and therefore at the nanotube/nanofibergrowth temperature T_(g), is formed (where T_(g)<T_(a)). Consequently,when catalyst spots are subsequently used, these do not react with thealloy formed beforehand and they allow effective catalyticnanotube/nanofiber growth.

According to a variant of the invention, the process includes thedeposition of a catalyst layer made of the second material on thesurface of the alloy layer and then the local etching of said catalystlayer so as to define the catalyst spots.

Advantageously, resist spots may be produced beforehand on the alloylayer.

According to a variant of the invention, the first material and thesubstrate are of identical nature.

According to another variant of the invention, the first material andthe substrate are of different nature. In this case, advantageously ifthe upper layer of first material has a first number of atoms N_(M), andthe layer of second material has a second number of atoms N_(A), byadjusting N_(M)/N_(A)<x/y where x and y are molar fractions of the alloyM_(x)A_(y), it is possible during formation of the alloy with the secondmaterial in excess (relative to the formation of the alloy) to formcatalyst spots of said second material directly. In this case, it ispossible to dispense with the subsequent deposition of a catalyst layerin order to form catalyst spots for the purpose of formingnanotubes/nanofibers.

Advantageously, the first material may be silicon or a metal.

When the alloy layer is obtained after a layer of second material hasbeen deposited on the upper layer at the surface of the substrate andthe whole assembly has been annealed, this alloy layer may typicallyhave a thickness of between about ten nanometers and about one hundrednanometers.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein the preferred embodiments of the invention areshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription thereof are to be regarded as illustrative in nature, andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages will becomeapparent on reading the description that follows and with the aid of theappended figures in which:

FIGS. 1 a-1 d illustrate the steps of a catalytic nanofiber/nanotubegrowth process according to the prior art;

FIGS. 2 a-2 e illustrate the steps of an example of a nanotube/nanofibergrowth process, according to the invention; and

FIGS. 3 a-3 c illustrate the steps of a second example of ananotube/nanofiber growth process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the nanotube/nanofiber growth processcomprises the production of a barrier layer forming a barrier withrespect to a catalyst layer, necessary for growth of thenanotubes/nanofibers.

According to a variant of the invention, the process includes theproduction of a cobalt layer of about fifty nanometers on the surface ofa silicon layer, in order to produce the CoSi₂ alloy.

FIG. 2 illustrates a first example of a process according to theinvention, in which the substrate and the material of the upper layerare of different nature. (However, according to other variants of theinvention, the substrate S may itself be made of the material M).

In a first step, the barrier layer is produced by predepositing a layer12 of material A on the surface of an upper layer 11 of material M,itself on the surface of a substrate S (FIG. 2 a).

An annealing operation is then carried out at a temperature T_(a), whichallows the formation of a layer 13 of alloy M_(x)A_(y) (FIG. 2 b).

Conventionally, a layer 14 of resist 2 is then deposited, and etched. Alayer 15 of catalyst material A is then deposited (FIG. 2 c). Afterremoval of the resist and excess catalyst material A (FIG. 2 d), spots16 of catalyst A are defined. The growth of the nanotubes 18 of amaterial B is then carried out at a temperature T_(g) below thetemperature T_(a) (FIG. 2 e), it being possible for the material B to beof the type C, SiC, BN, etc.

FIG. 3 illustrates a second example of a process according to theinvention in which the judicious choice of the amounts of catalystmaterial and of first material allow a layer of alloy M_(x)A_(y) andspots of catalyst of material A to be formed simultaneously.

As an example, it may be mentioned that a layer of silicon material Mwith a thickness of 185 Å and a layer of nickel material A with athickness of 100 Å, form at 750° C., a uniform layer of alloy NiSi. Asilicon defect or an excess of nickel makes it possible at the this sametemperature, to form a layer of alloy NiSi with residual surface spotsof Ni that can be directly used for growing nanotubes.

Thus, in FIG. 3 a, as in the example illustrated in FIG. 2 a, a layer 11of material M is deposited on the surface of the substrate S, followedby a layer 12 of material A on the surface of the layer 11. Thematerials M and the substrate S must be of different nature in order tokeep the material A in excess relative to the material M.

The annealing operation allows the simultaneous formation of a layer 13of alloy M_(x)A_(y) and of catalyst spots 17 corresponding to the excessof material A relative to the material M during formation of the alloy(FIG. 3 b).

The process then continues conventionally with the growth ofnanofibres/nanotubes 18 from said catalyst spots (FIG. 3 c).

EXAMPLE OF A NANOTUBE GROWTH PROCESS ACCORDING TO THE INVENTION 1stExample

Material M: Silicon

Material A: Nickel

In the case of a silicon substrate or of a thin silicon layer depositedon a substrate, a thin nickel layer is deposited on the silicon. Anannealing operation at 750° C. is then carried out so as to provide thecompound NiSi.

The addition of platinum prevents the formation of the alloy NiSi₂ andtherefore allows only the compound NiSi to be obtained (J. F. Liu etal., J. Appl. Phys. Vol. 90 p. 745 (2001)). The NiSi alloy thenconstitutes an effective diffusion barrier against nickel if thenanotube growth temperature is below 750° C. It should be noted that thelocalized and oriented growth of carbon nanotubes may be obtained at700° C. (K. B. K. Teo et al., Appl. Phys. Lett. Vol. 79 p. 1534 (2001)).

It is also possible to carry out the annealing at 850° C. so as to formNiSi₂, which constitutes a diffusion barrier for nanotubes/nanofiberswhose growth temperature is below 850° C.

A higher nanotube growth temperature (˜800° C. instead of 700° C.)generally allows nanotubes to be obtained that are of better crystallinequality and therefore characterized by better electrical properties.

After producing an NiSi diffusion barrier (formed at 750° C.) or anNiSi₂ diffusion barrier (formed at 850° C.), it is then possible tocarry out the localized and oriented growth of carbon nanotubes at atemperature of 700° C. In order to grow a single nanotube per catalystspot, nickel spots having a diameter of the order of 100 nm and athickness of 10 nm are defined by lithography. The growth can then becarried out at 700° C. in a CVD reactor enhanced by a DC plasma with avoltage of around 600 volts. A gas mixture containing acetylene andammonia (with ˜20% acetylene) at a pressure of about 5 torr then makesit possible to obtain oriented and selective growth of carbon nanotubes(K. B. K. Teo et al., Appl. Phys. Lett. Vol. 79 p. 1534 (2001)).

2nd Example

Material M: Silicon

Material A: Cobalt

In this second example, a cobalt layer is deposited on the surface of asilicon layer. To obtain a uniform CoSi₂ alloy and therefore to preventthe formation of the CoSi phase, the annealing may advantageously becarried out at a temperature above 600° C.

A thickness of around 30 to 60 nm of cobalt makes it possible to obtain,after annealing at 800° C. the CoSi₂ alloy (Y. J. Yoon, J. Vac. Sci.Technol. B17 p.627 (1999)). This compound formed at 800° C. then becomesan effective diffusion barrier for the cobalt catalyst, if the nanotubegrowth temperature is below 800° C.

3rd Example

Material M: Silicon

Material A: Iron

An FeSi₂ barrier layer may advantageously be formed by annealing an ironlayer on the silicon surface at 700° C. This barrier layer may be usedfor the growth of nanotubes/nanofibers at temperatures below 700° C.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto affect various changes, substitutions of equivalents and variousother aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bythe definition contained in the appended claims and equivalents thereof.

1. A process for the growth of nanotubes or nanofibers on a substratecomprising: forming an upper layer consisting of a first material;formatting, on the surface of the upper layer, a barrier layerconsisting of an alloy of the first material and of a second material,said alloy being stable at a first temperature; formatting spots ofcatalyst consisting of the second material, on the surface of the alloylayer; and growing nanotubes or nanofibers using the spots of catalystat a second temperature below said first temperature.
 2. The nanotube ornanofiber growth process as claimed in claim 1, wherein the formation ofthe barrier layer comprises the deposition of a layer made of the secondmaterial on the surface of the upper layer made of the first material,followed by annealing at said first temperature.
 3. The nanotube ornanofiber growth process as claimed in claim 1, comprising the steps of:depositing a catalyst layer made of the second material on the surfaceof the alloy layer; and local etching of said catalyst layer so as todefine catalyst spots.
 4. The nanotube or nanofiber growth process asclaimed in claim 1, wherein the first material and the substratecomprise identical materials.
 5. The nanotube or nanofiber growthprocess as claimed in claim 1, wherein the first material and thesubstrate comprises different materials.
 6. The nanotube or nanofibergrowth process as claimed in claim 5, wherein with the upper layer offirst material having a first number of atoms N_(M), and the layer (12)of second material having a second number of atoms N_(A),the numbersN_(M) and N_(A) are such that N_(M)/N_(A)<x/y where x and y are molarfractions of the alloy M_(x)A_(y).
 7. The nanotube or nanofiber growthprocess as claimed in claim 1, wherein the first material is silicon ora metal.
 8. The nanotube or nanofiber growth process as claimed in claim7, wherein the second material comprises one of nickel, iron or cobalt.9. The nanotube or nanofiber growth process as claimed in claim 8,wherein the first material is silicon, the second material is nickel andthe NiSi alloy is formed in the presence of platinum.
 10. The nanotubeor nanofiber growth process as claimed in claim 1, wherein the layer ofsecond material has a thickness of between about 10 nanometers and 100nanometers.
 11. The nanotube or nanofiber growth process as claimed inclaim 7, wherein it includes the production of a cobalt layer of aboutfifty nanometers on the surface of a silicon layer, in order to producea CoSi₂ alloy.
 12. The nanotube or nanofiber growth process as claimedin claim 7, wherein it includes the production of an FeSi₂ alloy. 13.The nanotube or nanofiber growth process as claimed in claim 2,comprising the steps of: depositing a catalyst layer made of the secondmaterial on the surface of the alloy layer; and local etching of saidcatalyst layer so as to define catalyst spots.
 14. The nanotube ornanofiber growth process as claimed in claim 2, wherein the firstmaterial and the substrate comprise identical materials.
 15. Thenanotube or nanofiber growth process as claimed in claim 3, wherein thefirst material and the substrate comprise identical materials.
 16. Thenanotube or nanofiber growth process as claimed in claim 2, wherein thefirst material and the substrate comprise different materials.
 17. Thenanotube or nanofiber growth process as claimed in claim 3, wherein thefirst material and the substrate comprise different materials.
 18. Thenanotube or nanofiber growth process as claimed in claim 6, wherein thefirst material is silicon or a metal.
 19. The nanotube or nanofibergrowth process as claimed in claim 8, wherein it includes the productionof a cobalt layer of about fifty nanometers on the surface of a siliconlayer, in order to produce a CoSi₂ alloy.
 20. The nanotube or nanofibergrowth process as claimed in claim 8, wherein it includes the productionof an FeSi₂ alloy.